Pulse-echo position indicator



D. w, EPSTEIN PULSE-ECHO POSITION INDICATOR Filed June 26, 1942 'I Fiend;

Dec. 17, 1946. D, W EPSTElN 2,412,670

PULSE-ECHO POS ITION INDICATOR Filed June 26, 1942 2 Sheets-Sheet 2 34 CHW/ODL' )MY TUBE 52 1G. Z.

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Patented Dec. 17, 1946 PATENT ori-fica PULSE-ECHO POSITION INDICATOR David W. Epstein, Merchantville, N. J., assignor to Radio Corporation of America, a corporation i of Delaware Application June 26, 1942, Serial No. 448,663

- 8 Claims. 1

My invention relates to pulse-echo position indicating system and particularly to systems adapted to convey the position information to a remote point or central station. It will be described as applied toa position indication system-in which a directive transmitting antenna is rotated horizontally to sweep a radio beam through a horizontal plane while pulsingthe beam. 'I'he radio pulse is reflected if it strikes The reflected pulse is picked up by a receiver located near the pulse transmitter and the distance of the reflecting object is determined by the time elapsed between transmission of the initial or time-reference pulse and receptionof the reected pulse. The direction of the reilecting object is determined also by employing at the receiver a cathode ray indicator tube in which the cathode ray is deflected radially and at the same time, rotated in synchronism with the rotation of the transmitting antenna.

It may be desirable to install a plurality of stations of the above described type spaced apart the proper distance to give protection over a large geographical area. In that case it may be desirable to convey the information from all pulseecho stations to a central station where one person has before him a view of all the position indicating views or patterns that are present at the individual stations. A dimculty in doing this is that such infomation ordinarily occupies a fairly wide frequency band, from 100 to 60,000 cycles per second, for example. This band is too wide for transmission over the usual telephone, tele.- graph or wire line.

An object of the present invention is to provide an improved method of and means for reducing the above-.mentioned frequency band, preferably enough to permit transmission of the position indication information over a wire line to a central station.

Another object of the invention is to provide an improved method of and means for conveying information from a radio pulse-echo position indicator to a remote point.

A further object of the invention is to provide an improved means for and method of producing an orienting indication in a system of the abovedescribed type.

In one embodiment of my invention the position indication at the remote station appears on a cathode ray tube provided with a defiecting yoke that is rotated in synchronism with the rotation of the transmitter antenna.` 'I'he radial deection produced by this yoke is at a low frequency (Cl. Z50-1) 2 rate as compared with the rate at the indicator tube located at the transmitter; these rates are 30 per second and 600 per second, respectively, in the example describedl below. The signal for 5 pulsing or modulating the cathode ray of the indicator tube at the remote station (this signal corresponding to the reflected pulses applied to the indicator tube at the receiver which is usually located in the vicinity of the transmitter) isobtained by applying the reflected pulses to a second cathode ray tube at the transmitter to store them on a fluorescent screen and by deflecting the cathode ray of this tube along a line trace at the pulse rate. The resulting stored pulse appearing at a point on the line trace is scanned by a rotating disc having radial slits which scan the line trace at the low frequency radial deflection rate employed at the remotey indicator. When a scanning disc aperture'comes opposite the light spot on the line trace, a pulse of light is transmitted to a photoelectric cell to produce a corresponding electrical pulse. These pulses may be transmitted .over an ordinary wire line such as a telephone line or over a narrow frequency band transmitter to the cathode ray tube indicator at the remote or central station since their repetition rate in the example assumed is per second, instead of 600 per secondv as the transmitter station, with a corresponding reduction in 30 the frequency band.

The invention will be better understood from the following description taken in connection with the accompanying drawings' in which Fig. 1 is a diagram of a pulse-echo position `indicator system embodying the invention, Fig. 2 is a view of the scanning disc shown in Fig. 1,- and Figs. 3 to 8a are graphs which are referred to in explaining the invention. Like parts in the figures are indicated by similar reference numerals.

40 Referring to Fig. 1, a radio transmitter I0 is keyed at the rate of 600 pulses per second. for example, to radiate pulses of very short duration from a suitable directive antenna Il. Antenna Il may be a dipole in a parabolic reflector I2 or it may be of any other suitable design. It is rotated at 10 rotations per minute, forexample, in a horizontal plane about a vertical shaft I3. The shaft I3 is driven by a synchronous motor It through a gear box I6, the mechanical coupling being indicated by the broken lines. The motor I4 is connected to a cycle power line I6.' Suitable pulse keying means I1 is driven by a synchronizing pulse generator I8.

The receiver I9, which is located in the vicinity of the transmitter, receives 'the reected pulses of carrier wave frequency, demodulates them, and applies them to the control electrode .of a cathode ray indicator tube 2| having a fluorescent screen 22; Tube 2| is provided with a rotatable deecting yoke 23 of the magnetic type having slip rings 24 and 23 which are connected through brushes to a sawtooth generator 21. The yoke 23 is rotated synchronously with the antenne rotation by means of gears 23 and 23 which are mechanically coupled to the gear box I3.

The sawtooth generator 21 is synchronized with the radio pulse transmission by means of pulses from the synchronizing -generator I3 supplied over a conductor 3l. The deiiecting wave applied to the yoke 23 contains a direct current component such that the deection of the cathoderay is from the center of screen .22 toward the circumference. The cathode ray tube 2| may be biased nearly to beam current cutoii! in the absence of a received pulse or to some value less than maximum Ibeam current whereby a luminous spot appears on the screen 22 upon the reception of a puise. This spot has a circumferential position depending upon the direction of the reiiecting object from the rotating transmitter antenna and has a radial position (distance from center of screen) depending upon the distance of the reecting object from the transmitter antenna.

Ii an attempt were made to transmit the reected pulse, wnich recurs at 600 times per second, to a remote station over an ordinary wire line, for example, it would be found that the delity of transmissionwould be very poor as the line would not transmit the required frequency band. Therefore, I have provided means for reducing the frequency band required whereby infomation sent over a line to a central station will give on the indicator tube an acceptable indication of the position of the airplane or other refleeting object.

The frequency band reducing means comprises a cathode ray tube 32 having a iluorescent screen 30 on the enlarged end. Deflection of the cathode ray is provided by means of a pair of defiecting coils 33 which are supplied with sawtooth current from the sawtooth generator 21. The 600 cycle sawtooth current is synchronous with the keying pulses.

The tube 32 is properly biased nearly to beam cutoff, for example, so that a spot of light 35 appears on the screen 30 when a reilected pulse is applied to the control electrode of tube 32 over a conductor 4 I. This light spot appears at a point on the linear deflection path or trace indicated by the dotted line 34 and 'at a position determined by the time that the reected pulse appears upon the control electrode.'

A rotatable disc 42 having a plurality of radial slit apertures is located opposite the screen 30. In order to direct light from the spot 35 to a photoelectric cell 43,v a lens system 41 is provided to image the light spot 35 on a disc aperture. The

dise 42 is rotated te cause the apertures 4s to scan' the spot 35 at a low rate compared with the pulse rate, this being 30 per second in the example il- -lustrated whereby theirequency band is reduced yto 3%00 or 17g@ ot the original band width. The Vdisc may be driven by a motor 5I which may or rotated in synchronism with the antenna rotation. This may be done by a synchronous motor 51 that is driven synchronously from the telephone line as will be explained below. A sawtooth deilecting wave recurring 30 times per second is produced in the yoke 56 by a sawtooth wave generator 3| to deiiect the cathode beam radially. The generator 6I is held in xed time relation to the pulse transmission by synchronizing pulses transmitted over the wire line. It will be understood that atthe indicator tubes 2i and 54 the sawtooth sweep is so phased with respect to the transmitter pulse that the deiiection starts from the center of the fluorescent screen at the instant the pulse is transmitted. The uorescent screen of the tube 54 is indicated at 53.

The synchronizing pulses for controlling both the sawtooth generator 3i and the synchronous motor 5l are produced at a photoelectric cell 33 positioned opposite the scanning disc 42 whereby light from a source I1 falls upon the cell 83 each time a slit aperture 45 comes between the source 61 and the cell 66. A mask 65 may be provided to deiine an area of light opposite a disc aperture.

The resulting synchronizing pulses are amplified by an amplier 68 and are supplied to the mixing ampliiler 50.

Reference or north indicating" pulses 'il are also produced by means of a photoelectric cell 33 and a light source 1 I positioned on opposite sides of the scanning disc 42. The pulses from the cell 69 are ampliiied by an amplier 12 and are supplied through a biased amplifier 13 to the mixing amplier 50. A mask 14 limits the area of light opposite the synchronizing slit.

Fig. 2 indicates the relative radial positions of the photoelectric cells 43, 53 and 36 (which have been displaced toward the center of the scanning disc to avoid confusion in the drawings) and of their associated light sources while Figs. 3 to 4 show the photoelectric cell outputs and the way in which they are mixed. The rotation yof the scanning disc 42 is in the direction indicatedr by the arrow. At the instant the disc 42 is in the position illustrated, a synchronizing pulse (Fig. 3) is being generated and a north indicating pulse (Fig. 6) has just been generated. The line trace 34 and spot 35 (Fig. 4) have already been scanned and will now be scanned again. y

A S shown in Fig. 5, the synchronizing pulses are reversed in polarity by ampliner 33 and are mixed in amplliler 50 with the signal representative of spot 35. Also, the reference or north indicating pulse is combined with the signal (shown in Fig. 5) to produce the resultant signal shown in Fig. 7. This signal is sent over the wire line to the central station.

At the central station the signalis ampliiled by ampliilers I1 and 13 and is applied to the control electrode of cathode ray tube 54 with the synchronizing pulses of negative polarity. The same signal, but preferably of opposite polarity, is supplied to a synchronizing pulse separator 18 whereby synchronizing pulses may be supplied to the deilecting circuit 3| and to a tuned ampliiler 3l which drlve's the motor 61. A diilerential gear 32 permits phasing of the yoke 53 by turning a crank 33.

The north indicating pulse appears on the screen of tube 54 only when the antenna il is pointing north (or is pointing in some other predetermined direction). This is accomplished by driving a switch 33 from the gear box I3 or from antenna shaft i3 so that the switch 33 is closed only when antenna il is pointing in the desired direction. When the switch 88 closes it completes a circuit through a conductor 81 to unblock a relaxation oscillator much as a multivibrator I9 whereby multivibrator pulses 9| (Figs. B and 8a) are produced. When the switch 86 is closed it connects the cathode of a multivibrator tube to ground to initiate a pulse 9|.

Multivibrator 89 is adjusted so that a pulse 9| which is short enough to add only one "north indicating pulse 'l0 is added thereto in the biased amplifier 'I3 as shown in Fig. 8. As shown in Fig. 8a, the antenna switch 86 is opened before another multivibrator pulse 9| indicated in dotted line in Fig. 8a can occur. As shown in Fig. 8, the

amplier 'I3 is biased sulciently beyond cutoi so that a north indicating pulse is passed to the mixing amplier 50 only upon the occurrence of a multivibrator pulse 8|.

The persistence of fluorescence of the screen 83 and of the screen 22 of tube 2i should be several seconds, preferably six seconds where -the yoke 2. In a system for reducing the frequency band required to transmit the information contained in a wave consisting of periodically recurring electrical pulses each of which occurs at a certain time after the occurrence of a time-reference pulse, means comprising a cathode ray tube and a screen associated therewith for storing an image of said nrst pulses on said screen with said image spaced from a time reference point by an amount determined by sai certain time, said means including deilecting m ansior producing a straight line trace'on. said screen, and means for scanning said trace at a slower rate' than that 4oi the iirst pulses to convert said stored image to pulses recurring at said slowerrate, said scanning means comprising a scanning disc having a plurality of radial slits therein.

3. In a radio system, means for transmitting I' pulses to an object whereby they are reflected rotates 10 R. P. M. (i. e., once in 6 seconds; The

persistence of fluorescence of the screen 30 of the tube I2 should be at least l/too second (the time of one deflection) and preferably is about 1/ao second in the example shown for good signal to noise ratio. The fluorescence decays gradually,

as is well known, but a satisfactory screen is one in which the light falls to one-half maximum intensity in lsu second, for example.

From the foregoing it will be apparent that the pulse produced in the photoelectric cell I3 oc- -curs at a time following the instant of pulse M will be the same las on the indicator tube 2|.

It will also be seen that the north indicating pulse is obtained, without the use of any mechanical connection between the rotating antenna and the scanning disc 42. This may be an important advantage as the frequencyreducing unit comprising disc 42 may be located several hundred feet from the antenna. A further advantage is that with the proposed reference or north indicating system the scanning disc 42 itself need not be held in synchronism with the rotating antenna. A still further advantage is that .by means of my system a very exact timing of the north indicating pulse is obtained at the indicator tube 5I.

I claim as my invention:

l. In a system for reducing the frequency band required to transmit the infomation contained in a wave consisting of periodically recurring electrical pulses each of which occurs at a certain time after the occurrence of a time-reference pulse, means comprising a cathode ray tube and a fluorescent screen associated therewith for l storing an image of said rst pulses on said nscreen with said image spaced from a time reference point by an amount determined by said certain -time, said means including deilecting means for from said object, said pulses lrecurring at a certain rate, means for receiving said reflected pulses inthe region of the transmitting means, there being a certain time interval between said transmission and reception, a cathode ray tube having a uorescent screen that has persistence of fluorescence, means for defiecting the cathode ray along a linear trace on said screen at said certain rate, means for modulating said cathode ray by said reected pulses whereby an image thereof is stored on said'screen, means for scanning said screen at a lower rate than said certain rate and for simultaneously taking signal oft' said screen at said lower rate to produce pulses at said lower rate, said last means including a scanning disc having a plurality of circumferentially spaced apertures located to rotate the apertures'past said trace to scan it lengthwise, a cathode ray indicator tube having a beam control element, meansx recurring at a certain rate, means for receiving said reflected pulses in the region of the transmitting means, there being a certain time interval between said transmission and reception, a cathode ray tube having a storage screen, means for detlecting the cathode ray along a straight line trace on said screen at said certain rate, means for modulating the said cathode ray by said reflected pulses whereby an image thereof is stored on said screen, means comprising a rotatable disc having circumferentiaily spaced slits forscanning said trace at a lower rate than said certain rate andfor simultaneously taking signal' off said screen at said lower rate to produce pulses occurring at said lower rate, a cathode ray indicator tube which contains means for generating an electron beam, means for applying said lower rate pulses to said indicator tube to modulate said beam, means for radially deiiecting the electron beam of said indicator tube, means for rotating the plane of said radial deection in synchronism with the rotation of said antenna, and means for producing said radial deilection at said lower rate with the deflection from center starting substantially at the instant of a pulse transmission.

5. In a system for indicating the position of an object. means for transmitting pulsesto said obiect from a directive rotating antenna whereby /sntna-evo they are reflected from said object, said pulses recurring at a certain rate, means for receiving said reflected pulses in the region of the transmitting means, there being a certain time interval between said transmission and reception, a cathode ray tube having a storage screen, means for deflecting the cathode ray along a straight line trace on said screen at said certain rate, means for modulating said cathode ray by said reflected pulses whereby an image thereof is stored on said screen, means comprising a rotatable disc having circumferentially spaced slits for scanning said trace at a lower rate than said certain rate and for simultaneously taking signal o1! said screen at said lower rate to produce pulses occurring at said lower rate, a cathode ray indicator tube which contains means for generating an electron beam, means for applying said lower rate pulses to said indicator tube to modulate said beam, means for radially deflecting the electron beam of said indicator tube, means for rotating the plane of said radial deflection in synchronism with the rotation of said antenna, means for producing said radial/deflection at said lower rate with the deection from center starting substansaid reflected pulses in the region of the transmitting means, there being a certain time interval .between said transmission and reception, a cathode ray tube having'a fluorescent screen that has persistence of fluorescence, means for deflecting the cathode ray along a straight line trace on said screen at said certain rate, means for modulating said cathode ray by said reflected pulses whereby an image thereof is stored on said screen, means comprising a rotatable disc having circumferentially spaced slits for scanning said trace at a lower rate than said certain rate and for simultaneously taking signal of! said screen at said lower rate to produce pulses at said lower rate, a cathode ray indicator tube which contains means for generating an electron beam, means for applying said lower rate pulses to said indicator tube to modulate said beam, means for radially deiiecting the electron beam of said indicator tube, means for rotating the plane of said radial deflection in synchronism with the rotation of said antenna, means for producing said ,radial deiiection at said lower rate with the de- :Election from center starting substantially at the instant of a pulse transmission, means including a photoelectric device located adjacent to said scanning disc for producing synchronizing pulses for synchronizing the deflection of said indicator tube, and means including a second photoelectric device located adjacent to said scanning disc and circumferentially displaced with respect to the other photoelectric device for increasing the intensity of the indicator tube beam momentarily in response to the rotating antenna reaching a predetermined point in its rotation.

'7. In a system for indicating the position of an object, means for transmitting pulses to said object from a directive rotating antenna whereby they are reected from said object. said pulses recurring at a certain rate, means for receiving 8 said renected pulses in the region of the transmitting means, there being a certain time interval between said transmission and reception, a l

cathode ray tube having a storage screen, means for deilecting the cathode ray along a recurring path or trace on said screen at said certain rate. means for modulating said cathode ray by said reilected pulses whereby an image thereof is stored on said screen, means .comprising a rotatable scanning disc having a plurality of apertures for scanning said trace at a lower rate than said certain rate and for simultaneously taking signal oir said screen at said lower rate toiproduce pulses occurring at said lower rate. .a cathode ray indicator tube which contains means for generating an electron beam, means for applying said lower rate pulses to said indicator tube to modulate said beam, means for radially deiiiecting lower rate with the deflection from center starting substantially at the instant of a pulse transmission,r and means for increasing the intensity of the indicator tube beam momentarily in response to the rotating antenna reaching a predetermined point in its rotation, said last means including a photoelectric device and light source for producing a reference voltage pulse in response to a scanning aperture moving across the path of light from said light source to said photoelectric device, a biased amplifier tube, switch means for producing a voltage pulse in response to said antenna reaching said predetermined point, and means for utilizing said last pulse to unblock said biased amplifier momentarily to pass one of said reference pulses to said indicator tube.

8. In a system for indicating the position oi' an object, means for transmitting vpulses to said object from a directive rotating antenna whereby they are reflected from said object, said pulses recurring at a certain rate, means for receiving said reflected pulses in the region of the transl mitting means, there being a certain time interval between said transmission and reception, a cathode ray tube having a storage screen, means for deilecting the cathode ray along a recurring tatable scanning disc having a plurality of apertures for scanning said trace at a lower rate than said certain rate and for simultaneously taking signal oii' said screen at said lower rate to prof duce pulses occurring at said lower rate, a cathode ray indicator tube which contains means for generating an electron beam, means for applying said lower rate pulses to said indicator tube to modulate said beam, means for radially deflecting the electron beam of said indicator tube, means for rotating the plane of said radial deflection in synchronism with the rotation of said antenna, means for producing said radial deflection at said lower rate with the deection-from center starting substantially at the instant of a pulse transmission, and means for increasing the intensity of the indicator tube beam momentarily in response to the rotating antenna reaching a predetermined point in its rotation, said last means including photoelectric means for producing a reference voltage pulse each time a scanning aperture passes a certain point, a biased amplilast pulse to said relaxation oscillator to unblock it momentarily, and means for applying said reference pulses and said relaxation oscillator pulses in additive relation to said biased amplier and with such amplitudes that only the reference pulse which is added to a relaxation oscillator pulse is passed by said biased amplier.

DAVID W. EPSTEIN. 

